Process of and apparatus for digesting fibrous materials



Oct 1, 1940- A. D. NERRILL PROCESS OF AND APPARATUS FUR DIGESTINGFIBROUS MATERIALS Filed June 27, 1955 amen Mot! AlberDMemdL PatentedOct. 1, 1949 STATES PATENT OFFICE Albert D. Merrill, Watertown, N. Y.

Application June 27, 1935, Serial No. 28,719

7 Claims.

This invention relates to improvements in a process of and apparatus fordigesting fibrous material.

It has been recognized heretofore that the 6 quality and yield of pulp,produced for example by the sulphite process, is dependent to noinconsiderable degree upon the extent to which the chips are penetratedwith acid prior to the actual cooking. If the chips are not thoroughlyper- 10 meated with acid before the entire mass is raised to reactiontemperature, differential cooking obtains with the result that thecenters or cores of the chips may be undigested. This conditionofvariation in the cooking is likewise accentuated when there is a markeddifference in temperature in different parts of the digester.

To overcome these objections it has heretofore been proposed to secure athorough penetration of the chips with acid by compacting the chips withhot acid, maintained below reaction temperatures, and at a highhydrostatic pressure. Such prior proposals have also included thesuggestion of circulating the acid to and from the digester both duringthe penetrating or soaking period and the cooking period so as toestablish substantially uniform thermal conditions through the digesterand thus avoid differential delignification and produce a uniform pulp.

Such prior proposals, while very effective, re-

quired considerable apparatus, among other things a relatively largehigh pressure accumulator for storing the body of hot acid for thesoaking period. I

A major object of the present invention is to devise a simplifiedprocess for "uniformly digesting a mass of fibrous material. I

A further object is to provide a process of delignifying fibrousmaterials in which the material is uniformly saturated with hot acidprior to cooking.

Yet another object is to provide an improved digester-storage tanksystem in which optimum heat recuperation is secured.

With these and other equally important objects in view the inventioncomprehends the concept of achieving the advantages of a thoroughpenetration with high temperature-high pressure acid, coupled with theadvantages incident to uniform thermal conditions during treatment andto se- 50 cure the enumerated advantages in a greatly simplifiedapparatus.

As indicated hereinbefore, it has been proposed to pre-circulate hothigh pressure acid through a mass of chips in a digester for the purposeof insuring a thorough penetration of the acid in the chips. Such priorprocesses were advantageously operated by employing a high pressureaccumulator in the system, this accumulator functioning to recover thethermal and chemical values of the liquor and in addition providing areadily available supply of hot preconditioned acid for the soaking andcooking steps.

It has now been found that such major advantages can largely be securedin a special design simple unit comprising essentially a digester orbattery of digesters and a low pressure storage tank or tanks.

,The present invention is an improvement on the process and apparatusdisclosed in the United States patent to T. L. Dunbar, Number 1,974,420.

Considered more specifically the present process involves a treatment offibrous materials with a suitable digesting agent, such as a typicalbisulphite liquor, in such a manner as to secure a number of advantageswith a minimum of apparatus. Such process thus involves charging thedigester with chips, forcing a quantity of acid from a storage tank tothe digester while heating the acid in transit with steam and/ordirectly or indirectly with hot liquor and gas obtained from anotherdigester of the unit. The entering preheated and preconditioned acid isutilized to displace air from the digester, thus substantiallyprecluding subsequent oxidation reactions. After the digester has beensubstantially or completely filled with hot acid an additional quantityof acid is forced into the digester so as to establish a highhydrostatic pressure on the liquor. In these circumstances, therefore,the hot acid readily penetrates the chips. To equalize the temperaturethroughout the digester and further to facilitate penetration of theacid in the chips, the acid is recirculated to and from the digester.During this recirculation the acid may be further heated, if desired ornecessary, to bring it up to the optimum penetrating temperature.

After such preliminary soaking and precirculation period the temperatureand pressure conditions within the digester are adjustedto initiatecooking. Duringthe cooking period hot gases-and liquorare relieved fromthe digester and the thermal and chemical values are recovered in thesystem, in part in a digester undergoing filling orsoaking and in partin the acid storage tank.

In order more clearly'to explain the underlying principles of theinvention, a typical physical embodiment is shown in the single figureof the ac'-' companying drawing.

As shown in the drawing, the unit may comprise one or preferably aplurality of digesters l associated with an acid storage tank or tanks 2of sufficient capacity to fulfill the requirements for the unit. Thestorage tank is supplied from the acid make-up unit, not shown.

The digesters may be of the type common in the art constructed of asuitable metal and lined with an acid resistant ceramic or equivalentmaterial. It is provided with a removable manhole cover 3 and the usualblowoff elbow 4. Connected to the upper top of the digester is the toprelief line 5 having a control valve 5. The portion of the line 5terminating within the digester is provided with a suitable strainermeans, as well understood in the art. The other end of the relief linemay be connected to a header 8. Passage of relief fluids into the headermay be controlled by the valve 6'. An air vent 1, provided with thecontrol valve I, may be suitably connected with the upper portion of thedigester, as for example through the line 5.

The acid storage tank 2 may be placed in communication with the digesterthrough the line 8. Fluids passing from the header 6 through line 8 maybe passed directly to the storage tank through the drop leg 9,controlled by the valve 8', or optionally, through the cooler l0,controlled by valve l, and thence through the drop leg H into thestorage tank to be mixed with the acid liquor therein. Lines 9 and IIpreferably-terminate at a low point in the tank.

The digester is likewise provided with a'side relief line l2 sopositioned on the digester as to 'establishthe desired liquid levelduring the cooking operation. This line is provided with a control valvel2 and the line terminates, as shown, at a low point within theseparator l3. By-pass line ll connects the side relief line l2 with thetop relief so that such side relief, in a manner more fully to bedescribed, may be passed to the storage tank 3; by way of conduit 8.

Separator l3 may be of any suitable size and construction and is made ofor lined with a suitable acid resistant material. The separator isprovided with a knock-back or reflux coil I5 provided at its efllux'endwith a pressure control valve l5. This valve is utilized to. maintainany desired pressure in the separator and thus control the extent ordegree of separation of free gas from the side relief liquor. Thisknock-back coil communicates with the gas discharge line l6. Line l6connects with the drop leg 9 through an injector fitting H. A valve I6is provided in the line ii to control the passage of gas to the dropleg. It will be understood that with such a connection liquor passingdownwardly through the line 9 imposes a suction on the line l6 andthusserves to withdraw and condense or absorb the gas from such line. Itwill be understood also that the line l8 may be connected through aninjector type of fitting to the drop leg II from which the coolerliquids flow, thus increasing the absorption of as.

A by-pass line I8, controlled by valve l8, may be connected between linel8 and line 8 so that overhead gases from the separator may be admixedwith fluids passing through line 8 to the cooler III. The connection [9between lines i 8 and 8 may be of an educator type so as to establish asuction on line l8 and thus facilitate the withdrawal of gas from thisline.

Acid is fed from the storage tank to the digester through the line 28provided with the control valve 20'. Line 28 communicates with theheader 2| and thence through the branch line 22, controlled by valve22', to the blowoff elbow or any other suitable low point on thedigester.

As indicated hereinbefore, the acid is heated in transit from thestorage tank to the digester for the purpose of raising it to theoptimum soaking or penetrating temperature. As has been explainedheretofore, a desideratum in the present invention is to contact thechips in the digester with hot high pressure liquor while maintainingthe temperature of the liquor below reaction temperatures for thepurpose of securing maxi- 'mum penetration with substantially noreaction.

heat of the system.

As shown, the separator I3 is provided with a bottom drawofjf line 23,the passage of hot side relief liquor through which is controlled byvalve 23'. This liquor is used to directly and/or indirectly heat therelatively cool acid passing from the storage tank to the digester. Thusthe line 23 is connected by a branch, controlled by valve 23", to theindirect heat exchange 24 and this liquor constitutes the heating mediumin such heat exchange. Acid liquor from the storage tank passing throughline 20 enters the indirect heating coil 25 and is thus indirectlyheated by the hot relief liquor.

The present system may also be employed not only to preheat the soakingliquor but also to precondition it. For this purpose the branch line 26is provided. This line is connectedat one end into line 20 betweencontrol valve 20' and pump 21 or, if desired, directly to the digester.The flow of hot side relief liquor into line 28 at this point iscontrolled by valve 26. The branch line 28, controlled by valve 28',provides for the passage of hot side relief liquor into the acid feedingline 2!! beyond the eductor 28. In this manner hot relief liquor may becommingled or admixed with the relatively cooler. acid at the highand/or low pressure side of feed pump 21. Fresh acid from the storagetank therefore may be preheated and preconditioned by direct admixturewith hot side relief liquor from the separator l2.

Increased heating economies and additional preconditioning of thesoaking liquor may be secured by admixing gas evolved during the cookingoperation with the fresh acid. As shown, this may be accomplished byinserting the eductor 28 in the feed line and connecting the suctionside of the eductor with the line 30. Line 38, controlled by valve II',is connected, as shown, with the overhead gas line l8. When valve 28' isopened hot gases are aspirated from line It, under the action of thehigh velocity acid passing through the eductor, and such gases areintimately admixed and-absorbed by the acid. By controlling the valves28, 28 and 25', the degree of preheat imparted to the cooler acid fromthe storage tank may be accurately controlled. Similarly, by controllingthe quantity of the hot relief liquor passing through lines 28 and 28,the degree of heat imparted to the acid may be accurately controlled soas to secure the maximum absorption of gas passing into the stream fromline 88.

The relatively cool acid flowing from the acid storage tank through line28, preheated by indirect and direct exchange with hot relief gases andliquids, is further preheated in transit by steam. As shown, the steamline header 48 is provided with a branch line H, controlled by valve ll.

This branch line discharges into the blowofi elbow. If desired,additional branch lines 4i may be connected into line 26 at any desiredpoint between the storage tank and the digester to achieve the maximumdegree of preheat with optimum concentration of free gas. Preferably, asshown, the final heating with steam is achieved after the preliminarypreheating and preconditioning by the hot digestion fluids. Steam lineQ!) is also provided with branches 7.72, controlled by valves 42, whichconnect into the lower conical portion of the digester in the mannerwell known to those skilled the art.

The hot side relief liquor, after passing through the heat exchanger 24,may be picked up by pump 50 and forced through line 5! either directlyinto the acid storage tank to branch 52 or to line 53 through the coolerin and thence to the storage tank. If desired, the connection 54 betweenline 53 and the header 8 may be of the injector type of fitting so thatthe liquid forced through line 53 may be utilized to condense and absorbfree gases in the relief fluids flowing through line 8'. The control ofpassage of the side relief liquor through the line 52 or 53 iscontrolled by valves 52' and 53 respectively.

As already indicated, according to the present invention relatively coolacid from the acid storage tank is first preheated and preconditionedfor employment as a high temperature soaking liquor in the digester. Inorder to secure optimum penetration the liquor is preferablyrecirculated to and from the digester. Such recirculation tends toequalize the temperature throughout the digester and this uniformity oftemperature, coupled with the fact that the acid is at high temperatureand pressure, insures complete and uniform saturation of the chips withhot acid.

To secure this result the digester, as shown, is provided with arecirculation circuit. This may comprise the recirculation line 60,controlled by valve one end of which terminates in a perforated section62 within the digester and the other end of which connects with thesuction side of the pump 63. The high pressure side of the pump isconnected through line 64 to the bottom portion of the digester.Interposed in this line is the heat exchange means 65. This may be anindirect heater, the heating medium of whichmay comprise steam, hotspent liquor and the like. If

desired, in lieu of and in addition to the indirect 7 be maintained orincreased during the soaking heat the acid in another digesterundergoing.

period by adding S02 gas or its'equivalent at this .time. For thispurpose SO: gas may be withdrawn from the container and passed throughline 61 and control valve 61 to the suction side of recirculating pump.By cdntrolsoaking. One method of accomplishing this, namely by admixingside relief liquor from separrator it with the stream of acid flowingfrom storage tank 2, has been described.

soalnng liquor during the precirculation period, that is to say duringthe period when the pump 83 is operating. To accomplish this the line I0is connected to the suction side of pump 53. This line may be providedwith a control valve l8 and connected at its other end to the header 6or to a'separate high pressure header where such is employed. In thesecircumstances when, during the precirculation period, the digestercomprises a closed cycle, that is to say shut oif from feed line 213,additional heat units may be imparted to the soaking liquor bywithdrawing hot side relief liquor and forcing it through line if! intothe recycle stream.

The operation of the process will have been appreciated from theforegoing description. In treating a given mass of material, the manholeIn addition side relief liquor may be admixed with the cover 3 isremoved and the digester filled with chips from a suitable hopper orconveyor with or without the employment of a chip packing device. Afterthe digester has been filled to the desired level with chips the coveris bolted on, Valves 5 and I are opened, valve 6, i2 and 60' beingclosed. Valve 20' is then opened and pump 21 operated to forcerelatively cool acid from the storage tank to the digester. In transitto the digester the acid is heated so as to raise the temperature tobetween '70 to or 0. As explained, this may be done, for example, byopening valve 26' and valve 23. In these circumstances pump 2'! takessuction. on line 26 and hot side relief liquor from the separator I3 isadmixed with the relatively cool acid flowing through line 20. Thispreheats and preconditions the cool acid. Coincidentally valve 30' maybe opened so that the high velocity stream of acid flowing througheductor 29 establishes a suction in line 30 and thus withdraws gas fromthe separator. This hot gas passing down through drop leg 30 isintimately mixed with and absorbed in the acid charge, additionallyheating the latter and increasing its free gas content. Likewise, ifnecessary to attain the desired soaking temperature, direct steam may beadmitted to the acid stream by opening valve 4 l and/or valves 42'. Thepreconditioned and partially preheated acid may then further be heatedby hot relief liquor entering the acid stream from pipe 28 andindirectly'by the hot relief liquor in the heat exchanger 24. y

In some circumstances it is not desirable to impart the maximum degreeof preheat tothe fresh acid until the digester is substantially filledso as to or the evolution of SO: gas during the filling operation. Asthe hot acid enters the digester at the bottom and flows upward,contained air is displaced and is discharged through the air vent 'I.This venting is continued until gas begins to discharge through 7 thevent, at which time valve 1' is closed and valve 6' opened. Thisoverhead gas may be absorbed in the acid within the storage tank bypreheated, in the manner described, that is to say by direct andindirect contact with hot relief liquid and by direct contact with hotgases and steam.

In the preferred operation the pump 21 is continued in operation until ahigh hydrostatic pressure is built up on the hot acid in the digesterand in the latter operation the pump is operated until a pressure of theorder of from ninety pounds or more is attained. During this period heatunits are imparted to the acid so as to raise it to the optimum soakingtemperature which, as noted hereinbefore, is just below reactiontemperatures and may be approximately from 100 to 110 C.

-- It will be understood that while number I digester is on the soakingperiod, the material in other digesters of the battery are undergoingcooking and hot relief fluids are being discharged therefrom to thestorage tank and/or the separator II in a manner more fully to bedescribed. After the desired penetrating pressure has been establishedon the volume of acid in the digester, valves 20, 28' and 23 areclosed'and pump, 21 stopped. It will be understood of course that insome installations having a large number of digesters the pump 21 andthe preheating assemblage, including the heat exchanger 24, may beoperated substantially continuously and the digester which is filled maybe cut off from the system by operating valve 22' interposed between theblowdown elbow 4 and header 2 I. After such filling of the digester,valve 60' is opened and pump. 63 operated so as to circulate the acid toand from the digester for the purpose, as explained, of equalizing thetemperature throughout. If, during the filling operation, the acidtemperature has not been raised to the desired degree, additional heatunitsmay be imparted to the mass by heating the recirculating stream inits passage through the heat exchanger 65 and/or by introducing directsteam. Likewise,

during this recirculation period the acid concentration may be increasedto any desired advantageous point by admitting S02 .gas from thecontainer 56.

Similarly, as described hereinbefore, the temperature of the soakingliquor may be increased by directlyadmixing hot relief liquor withdrawnfrom another digester. This may be done by opening the valve 70 andwithdrawing such liquor from manifold 6 through the line 10. It will beunderstood that a line may be provided from the bottom of separator l3to the suction side of pump 63 so as to withdraw hot side relief liquorfrom the separator and admix it with the recirculating stream of soakingliquor.

Thus acid at a temperature of from approxi-.

mately 80 to 110 C. and under high hydrostatic pressure may berecirculated through the mass of the chips in the digester until suchchips are thoroughly penetrated with the hot acid and are uniformlyheated throughout. In ordinary circumstances, using a typical sizedigester, such precirculation preferably is continued. for a period offrom one to four hours. In most circumstances, particularly where a highinitial temperature is utilized, a soaking period of two hours achievesthe advantages herein described.

After the precirculation-soaking period, the conditions within thedigester are adjusted to cause reaction of the bisulphite liquor withthe lignin for the purpose of delignifiying the fibers.

To initiate the cook valves 5', 6', l2 and i are opened and gas andliquor are relieved until the pressure and liquid level are reduced tothe desired extent. In these circumstances hot fluids pass through theheader 8 and cooler l0 and are admixed with the cooler acid in thestorage tank, thus preheating and preconditioning this acid. If desired,during this period valve l2 may be opened and valve I4 in line Il may beclosed, or only partially opened, so that some hot liquor is dischargedto separator I3 to be employed, as described, as a direct or indirectheating medium for the acid employed for soaking in another digester.

It has been found that a rather decided drop in pressure and liquidlevel is advantageous after a soaking period of the character described,as this drop materially increases the speed with which the mass may bebrought up to temperature. In the preferred operation, therefore, thepressure is dropped from the soaking pressure, of the order of 90 poundsmore or less, down to from 30 to 50 pounds more or less. This, it willbe appreciated, enables the employment of low pressure direct steam forcooking. The gases evolved as a result of this drop in pressure passoverhead through the header 8 and cooler Ill and are absorbed in theacid within the storage tank.

Upon such diminution in pressure and drop in liquid level, valves 42'are opened and valves 6' and I2 are closed. The admission of directsteam soon builds up the pressure within the digester and thetemperature is rapidly increased. When the pressure has been built up tothe normal desired operating level, of the order of 70 pounds gauge moreor less, this pressure may be maintained at substantially this value byopening valves l2, l4 and 6' and passing fluids to. the acid storagetank either by way of drop leg 9 or drop leg II or such pressure may bemaintained by closing valve 6' and I4 and relieving through valve l2 andline l2 into the separator I3.

During the cooking operation, in order to insure uniform thermalconditions throughout the mass, liquor may be recycled to and from thedigester through the recycle line 60 by operating the pump 62 in themanner described. In these circumstances, particularly when the indirectheat exchanger 65 is utilized, additional heat units may be imparted andthe extent of condensation and dilution of the acid commensuratelyreduced.

As is known, during the progress of the cook the liquid level tends torise due to the increased amount of condensate. The desired liquid'levelmay be maintained after cooking is started by operating the side reliefline in the manner de-' scribed. It will be understood that by propermanipulation of valves 5' and I2 the desired liquid level may bemaintained.

As is known as the reaction proceeds a considerable quantity of free gasis evolved and this tends to build up an excessive pressure in thedigester. In'the present operation such pressure is relieved by openingvalve 5' and passing gas through header 8 and valve ll through the tionof the cooking period is maintained at approximately 75 pounds andtemperatures at about 135 C.

Steam is shut ofi some time prior to the completion of the cook in themanner well known to those skilled in the art.

The pressure in the digester is then greatly reduced by relievingthrough header 8 down to approximately 20 or 30 pounds, after which thematerial is either discharged into the blow pit or is given apreliminary water wash in the digester and then discharged.

During the cooking operation, as noted here-' inbefore, optimumutilization of heat and chemical values is secured. The hot side reliefliquor passing to separator I3 is utilized as a preheating medium andthe gas evolved in the separator, to the extent determined by thesetting of pressure control valve i5, is utilized to preheat andprecondition the acid charge. The relief liquor passing through heatexchanger gives up a considerable quantity of-it's heat to the cooleracid and then is passed through line 5| either directly into the acidstorage tank through line 52 or by way of line 53 through the cooler IDto the storage tank. In some circumstances, as where a relatively highpressure is carried on the separator l3, such side relief may flow underits own pressure into the storage tank. In other circumstances, andespecially where a small size separator is employed, it is advisable towithdraw such hot liquor from the separator and through the heatexchanger at a relatively rapid rate. This may readily be done'byutilizing the pump 50 in line 5|. As noted hereinbefore, the owingstreams of liquid in the system may be utilized by employing a suitabletype of eductor fitting to more efiectively withdraw and absorb gasesevolved in the system. This result can be attained in the mannerdescribed by employing ejector types of fitting at 54 and Hi.

It will be appreciated that the present invention provides manyadvantages. The system comprises essentially, as the major units, onlythe digesters and storage tanks. The storage tank is made to perform aplurality of functions. In the first place it provides means ofaccumulating a large volume of acid and recovering some of the chemicaland heat values, of the system. Since the storage tank, however, isunder atmospheric pressure or at only slightly elevated pressure, thequantity of gas which may be absorbed and the degree of temperature riseis rather limited. In the present unit, however, additional absorptionand increased recuperation of heat units is achieved by utilizing theseparator l3 in conjunction with the acid preheating and preconditioningsystem 20-22-26-29 and 24. The present unit furthermore provides foroptimum soaking and accelerated reactions in a simple type of unit. Byutilizing the system as described the chips may be thoroughly penetratedwith hot high pressure acid maintained at a substantially uniformtemperature. In the present system although a relatively low pressureacid storage tank is employed, high hydrostatic pressure soaking withprecirculation is nevertheless achieved by circulating directly to andfrom the digester and cutting out the acid storage tank from the systemduring the high pressure soak.

By subjecting the fibrous material to the preliminary high pressure hightemperature soak the quantity of heat units required in the system ismaterially diminished. The system likewise advantageously and directlyutilizes the hot liquor withdrawn from one digester undergoing cookingand utilizes this in another digester undergoing soaking. By thisspecial correlation of contemporaneous soaking and cooking periods, inconjunction. with the other novel features of the unit, a large sizeaccumulator is rendered unnecessary, while nevertheless obtainingsubstantially the full benefit of such accumulator. In other words, thesoaking digester is made to act in part as a high pressure accumulator.This, in conjunction with the acid storage tank and the acid preheatingsystem, effectively con-= serves the heat and chemical values.

It will be understood that suitable control means, understood by thoseskilled in the art, will be provided for the unit. Thus, for example,although it is not shown, the digester may be provided with the usualtemperature and pressure indicating and/or recording devices. Similarlya thermostatic control may be associated with the digester and theheater 65 and operated to control the degree of heat imparted, by theheat exchange, during the soaking or cooking period. Likewise valve 61',associated with the source of S0 gas, may be controlled by a suitableelectrolytic element positioned within the digester so that the amountof S02 gas admitted to the recycle stream is controlled according to theconcentration of the liquor.

Therefore, while a preferred method of operation and type of apparatushas been described, it is to be understood that this is given merely asexemplifying the underlying principles of the invention and not as theexclusive method or means of eifectuating these principles.

I claim:

1. An apparatus of the character described comprising a digester and alow pressure acid storage tank, a valved conduit connecting the lowerportion of the digester, a pump in said conduit for pumping a stream ofacid from the tank directly to the digester, a valved side relief branchfor the digester, a valved top relief branch for the digester, meansconnecting the top relief to the storage tank and means including aseparator connecting the side relief to said valved conduit for directlyadmixing hot side relief with the stream of acid charged to thedigester.

2. An apparatus of the character described comprising an enlargeddigester and acid storage tank; an acid supply line extending from thetank to the digester; a side relief line connected with the digester andcommunicating with the storage tank, a separator and pressure reductionvalve in such line and means, including an eductor, connecting theseparator with the'said acid supply line.

3. In a process of the character described, utilizing a digester andacid-storage tank, that improvement which comprises withdrawingrelatively cool acid from the storage tank and forcing it directly intothe digester, separating side relief fluids into liquid and gaseouscomponents 'and utilizing the flowing stream of acid to withdraw andiurtherpreheating' the acid flowing to the digester by introducing steamdirectly therein.

5. In a process 01! the characterxdescribed,

utilizing a digester and low pressure storage tank, that improvementwhich comprises pumping a stream or acid from the tank to the digester,withdrawing reliei fluids from the digester in which cooking is takingplace, passing low presacid from a storage tank and forcing it into thedigester, then sealing the digester and forcing in additional acid tobuild up a high hydrostatic pressure in the digester; heating a streamof acid in transit from the storage tank to the digester to raise theacid temperature to the order of 70 C. and then recirculating the aciddirectly to and from the digester under the said a high hydrostaticpressure; then reducing the pressure and cooking the material within thedigester.

7. A process of digesting fibrous materials which comprises charging adigester with the material, withdrawing relatively cool acid from astorage tank and forcing it into the digester while displacing air fromthe digester with such entering acid; heating the stream or acid intransit to the digester by direct contact with hot fluids from anotherdigester, then sealing the digester and. continuing the introduction ofacid until a high hydrostatic pressure is established in the digester;then circulating the acid directly to and from the digester at such highhydrostatic pressure for a period of time sumciently prolonged tosubstantially completely saturate the material with acid and thenraising the mass to reaction temperatures and cooking the material inthe digester under a lower hydrostatic pressure.

' ALBERT D. MERRIIL.

